Bizarre Lunar Orbits

Nov.
6, 2006: Near the end of the mission of Apollo 16,
on April 24, 1972, just before returning back home to Earth,
the three astronauts released one last scientific experiment:
a small "subsatellite" called PFS-2 to orbit the
Moon about every 2 hours.

The
intention? Joining an earlier subsatellite PFS-1, released
by Apollo 15 astronauts eight months earlier, PFS-2 was to
measure charged particles and magnetic fields all around the
Moon as the Moon orbited Earth. The low orbits of both subsatellites
were to be similar ellipses, ranging from 55 to 76 miles (89
to 122 km) above the lunar surface.

Instead,
something bizarre happened.

The
orbit of PFS-2 rapidly changed shape and distance from the
Moon. In 2-1/2 weeks the satellite was swooping to within
a hair-raising 6 miles (10 km) of the lunar surface at closest
approach. As the orbit kept changing, PFS-2 backed off again,
until it seemed to be a safe 30 miles away. But not for long:
inexorably, the subsatellite's orbit carried it back toward
the Moon. And on May 29, 1972—only 35 days and 425 orbits
after its release—PFS-2 crashed.

Above:
An Apollo subsatellite leaves the Service Module,
an artist's concept. [More]

What
happened? The Moon itself plunged the subsatellite to its
death. That's the conclusion of Alex S. Konopliv, planetary
scientist at NASA's Jet Propulsion Laboratory in Pasadena.
He and several colleagues have been analyzing the orbits of
various Moon-orbiting satellites since PFS-2, notably the
1998–99 mission of Lunar Prospector.

"If
the Moon were a uniform sphere, you could have an orbit that
was perfect ellipse or circle," Konopliv explained. "The
Moon has no atmosphere to cause drag or heating on a spacecraft,
so you can go really low: Lunar Prospector spent six months
orbiting only 20 miles (30 km) above the surface."

So
why did PFS-2, which was inserted into an elliptical orbit that
originally carried it from 52 miles (97 km) to 66 miles (120
km), end up as a kamikaze blast of broken aluminum struts and
solar panels?

"The
Moon is extraordinarily lumpy, gravitationally speaking,"
Konopliv continues. "I don't mean mountains or physical
topography. I mean in mass. What appear to be flat seas of
lunar lava have huge positive gravitational anomalies—that
is, their mass and thus their gravitational fields are significantly
stronger than the rest of the lunar crust." Known as
mass concentrations or "mascons," there are five
big ones on the front side of the Moon facing Earth, all in
lunar maria (Latin for "seas") and visible in binoculars
from Earth.

The
mascons' gravitational anomaly is so great—half a percent—that
it actually would be measurable to astronauts on the lunar
surface. "If you were standing at the edge of one of
the maria, a plumb bob would hang about a third of a degree
off vertical, pointing toward the mascon," Konopliv says.
Moreover, an astronaut in full spacesuit and life-support
gear whose lunar weight was exactly 50 pounds at the edge
of the mascon would weigh 50 pounds and 4 ounces when standing
in the mascon's center.

Above:
Mascons on the Moon that make its gravitational field so lumpy,
as mapped by the Lunar Prospector mission, are shown in orange-red.
The five largest all correspond to the largest lava-filled
craters or lunar "seas" visible in binoculars on
the near side of the Moon: Mare Imbrium, Mare Serenitatus,
Mare Crisium, Mare Humorum and Mare Nectaris. Image reference:
Konopliv et al, Icarus 150, 1–18
(2001).

"Lunar
mascons make most low lunar orbits unstable," says Konopliv.
As a satellite passes 50 or 60 miles overhead, the mascons
pull it forward, back, left, right, or down, the exact direction
and magnitude of the tugging depends on the satellite's trajectory.
Absent any periodic boosts from onboard rockets to correct
the orbit, most satellites released into low lunar orbits
(under about 60 miles or 100 km) will eventually crash into
the Moon. PFS-2 released by Apollo 16 was simply a dramatic
worst-case example. But even its longer-lived predecessor
PFS-1 (released by Apollo 15) literally bit the dust in January
1973 after less than a year and a half.

So
what does this mean for eventual lunar exploration?

Be
careful of the orbit chosen for a low-orbiting lunar satellite.
"What counts is an orbit's inclination," that is,
the tilt of its plane to the Moon's equatorial plane. "There
are actually a number of 'frozen orbits' where a spacecraft
can stay in a low lunar orbit indefinitely. They occur at
four inclinations: 27º, 50º, 76º, and 86º"—the last one
being nearly over the lunar poles. The orbit of the relatively
long-lived Apollo 15 subsatellite PFS-1 had an inclination
of 28º, which turned out to be close to the inclination of
one of the frozen orbits—but poor PFS-2 was cursed with an
inclination of only 11º.

Alternatively,
if there are mission reasons for choosing a non-frozen orbital
inclination, plan to do frequent course corrections. Lunar
Prospector had to do a maneuver every two months to keep itself
in its initial circular orbit of 60 miles (100 km)—and more
often than once a month when it was orbiting at only 20 miles
(30 km) altitude. When its fuel tank was nearly empty, the
scientists knew its end was near, so they deliberately crashed
it on July 30, 1999, near the Moon's south pole to observe
its plume of lunar dust. After a year and a half, the Moon
had claimed the spacecraft for its own.

Lunar
mascons are a mystery. Although scientists
generally agree they resulted from ancient impacts billions
of years ago, it’s unclear how much of the excess mass
is due to denser lava material filling the crater or
how much is due to upwelling of denser iron-rich mantle
material to the crust. Regardless of composition or
origin, the mascons make the Moon the most gravitationally
"lumpy" body known in the solar system. Although
mascons also exist on Mars, none have been found on
Venus or Earth; those two larger planets, however, have
had an active tectonic (geological) past that has drawn
their crusts down into their interiors several times
in the past few billion years, homogenizing the distribution
of mass.

Details
about the subsatellites of both Apollos 15 and 16, including
their orbital parameters, appear on p. 5-5 of Apollo
16 Mission Report.

A
paper detailing the weird behavior of mascons on low
lunar orbits (of about 100 km altitude) by Alex Konopliv
and four coauthors is "Recent Gravity Models as
a Result of the Lunar Prospector Mission" published
in Icrarus, vol. 150, pp. 1-18, 2001 (available online
only by subscription).